1. Field of the Invention
The present invention relates to a sheet thickness measuring apparatus for measuring the thickness of a sheet of paper, plastics and the like by combinedly using a magnetic field utilizing sensor and an optical sensor.
2. Description of the Prior Art
Conventional sheet thickness measuring apparatus of such a type measures the thickness of a paper sheet 2 by scanning a combined sensor 5 including a magnetic field utilizing sensor 3 such as magnetic sensor or eddy current sensor and an optical sensor 4 along and above a paper sheet 2 disposed on a metallic back-up plate 1 as is schematically shown in FIG. 5 of the attached drawings. In principle, a distance l.sub.1 between the surface of the metallic back-up plate 1 and the sensor is measured by the magnetic field utilizing sensor 3 and a distance l.sub.2 between the upper surface of the paper sheet 2 and the sensor 4 is measured by the optical sensor 4. The thickness t of the paper sheet 2 at this time is determined by a formula as follows; EQU t=l.sub.1 -l.sub.2
However, it is difficult for such a conventional sheet thickness measuring apparatus to determine an exact sheet thickness since a distance value measured by the optical sensor 4 is unreliable if the sheet, the thickness of which is to be measured is transparent. Furthermore, even if the sheet to be measured is opaque, an actual measuring by such a conventional sheet measuring apparatus shows that measuring at an accuracy in the order of about 1/100 mm can be made without any problems while a problem will occur when measuring at an accuracy in the order of about 1/1000 mm. This problem is particularly remarkable when measurement is conducted while the sensor is moved with respect to the metallic back-up plate.
For example, by scanning the combined sensor 5 along and above the surface of the metallic back-up plate 1 on which no paper sheet 2 is disposed, outputs from the magnetic sensor 3 and the optical sensor 4 are recorded and plotted in FIG. 6. FIG. 6(A) shows an example of the output characteristics of the magnetic sensor 3 and FIG. 6(B) shows an example of the output characteristics of the optical sensor 4. Both output characteristics curves of the magnetic sensor 3 and the optical sensor 4 are expected to be same since both sensors measure the same surface. However, comparison of the curves in FIGS. 6(A) and 6(B) clearly shows that the output characteristics curve of the magnetic sensor 3 changes more largely and complicatedly than that of the optical sensor 4 does. Various experiments and references suggest that this is caused by an error of the magnetic sensor due to local magnetization of the metallic back-up plate and non homogeneity of material. If a magnetic material is used for the metallic back-up plate and a magnet or the like is placed on a part of the plate for locally magnetizing the plate to an approximate several gausses and then the sensor is scanned, the output from the magnetic field utilizing sensor exhibits a change of several tens .mu.m. The output from the magnetic field utilizing sensor exhibits a large change at some place even if inspection of the back-up plate by a Gauss meter having a resolution of 0.1 gauss suggests no change. Same problem will occur even if non-magnetic material is used for the metallic back-up plate and an eddy current magnetic sensor is used. It is deemed that such problem, in particular, an error caused by the magnetic field utilizing sensor hinders an improvement in measurement accuracy of such type of sheet thickness measuring apparatus.
It is an object of the present invention to provide a sheet thickness measuring apparatus which can overcome the aforementioned problems of the prior art.